Cable drives have taken on increased importance in mechanical transmissions used for small high-performance automated equipment. Increased exploitation of computer control places a higher value on light-weight, compact machines that react quickly to motor commands, and often these characteristics are achieved through the use of cable drives.
When properly designed, cable drives have high material strength, low weight, low velocity and torque ripple, no backlash, and low friction. Furthermore, they do not leak, do not require surface lubrication, and can be guided over long distances around pulleys through complex and twisting geometries. Cables and all other tension-element drives, such as tapes and belts, do not transfer power through compression or shear; and as a result they avoid added compliance and strength limitations found in gear teeth, linkages, and push rods, caused by bending moments or buckling. When designed for reliability, cable drives have a history of dependability in such demanding applications as aerial trams, ski lifts, cable cars, light-aircraft control surfaces, cranes, and elevators.
Generally, high performance is maintained only when the cables are pretensioned to roughly one-half of their maximum operating tensions. Several methods have been used to apply this pretension. All of the previous methods require substantial effort (30-120 minutes). One use of this new class of cable drive is in a robot arm used in oceanographic exploration. In such circumstances equipment reliability and maintainability are essential. For a given dive the equipment is transported by surface ships, where much of it is welded directly to the ship's deck to withstand rough seas. The ship then sails halfway around the world to the dive site. Once there, the manipulator arm is mounted on a submersible vehicle and lowered to some of the deepest ocean locations on earth. The exploration typically involves multiple dives. Each person on the vessel focuses on preparing the sub for its next dive. The cost per hour is enormous at this point.
During resurfacing, the cables on the robot manipulator are checked and retensioned if necessary while surface maintenance is performed on the submersible. If the cable maintenance exceeds a few minutes, it becomes the critical path. Using the current technique, the arm must be partially disassembled first. Next, three functions must be accomplished simultaneously and flawlessly: maintain existing tension in the cable circuit; keep the gap between the pinion halves below 0.001 inch by maintaining an axial force; and reset pretension.
These simultaneous functions require three wrenches and a screwdriver to be manipulated by three technicians. A lock nut or jam nut is loosened while the two parts of a split pinion are held against motion which would allow the cable to loosen and whip off the various drive pulleys. While holding the pinion against motion one must be counter-rotated against the other to pretension the cable. Then while the pinion halves are once again held motionless the lock nut must be tightened. If one of the technicians slips and either releases existing pretension or allows the 0.001-inch gap to open axially under cable stress, then the entire cable drive (generally multiple stages and many separate cables) has to be recabled. Recabling may require several technician-hours of labor. The entire recabling process still requires the final pretensioning.